The present invention relates to dual energy image registration, and more particularly, a variational method for dual energy image registration.
Conventional radiography (i.e., X-ray imaging) has been shown to have low sensitivity for detecting subtle details, such as lung nodules Accordingly, dual energy imaging can be used to evaluate such details. Dual energy imaging is a technique that acquires to images using low and high energy spectra, respectively. A sensor array is used to capture the rays that transverse through the subject. Since the attenuation coefficients of bone and soft tissue follow different functions of energy, the two images can be weighted and then subtracted to generate separate images for soft tissue and bone structure.
Well-known techniques for dual energy imaging include the Dual-Kilcolt (Peak) technique, the Single kV(p) Dual Filter technique, and the Sandwich Detector technique. The Single kV(p) Dual Filter and Sandwich Detector techniques acquire two images at one exposure by using some material to separate low and high energy image acquisition. These techniques are easy to implement, and any patient or anatomical motions have no effect on the results. However, due to material limitations and other factors, the separated images resulting from these techniques have low quality, and may have approximately three or four times as much noise as images resulting from the Dual-Kilcolt (Peak) technique. The Dual-Kilcolt (Peak) technique performs the entire image acquisition procedure at two different kV(p) levels (i.e., energy levels) in two sequential exposures. A time gap between the two exposures can range between 300 ms and 10 seconds, during which any motion of the patient or of anatomic structures within the patient may result in significant motion artifacts. Therefore, a registration method that is capable of compensating for any motion between the two images is needed.
Conventional image registration techniques cannot effectively register dual energy pairs for the following reasons. In X-ray images, all objects are transparent because X-rays are absorbed to different extents by different types of material as they pass through a patient. This means that one pixel in an image can contain portions of multiple anatomic structures, such as bones, the heart, the lungs, and other soft tissue. Therefore, each pixel may contain an arbitrary number of motions, such as heart motion and rib cage expansion due to aspiration. Conventional image registration techniques typically assume one motion per pixel. Furthermore, in dual energy imaging absorption, rates of bone and soft tissue do not relate linearly, and there is no objective mapping between intensity pairs for an image.